Understanding core tungsten (W) transport and control in an improved high-performance fully non-inductive discharge on EAST

Abstract

The behavior of heavy/high-Z impurity tungsten (W) in an improved high-performance fully non-inductive discharge on EAST with ITER-like divertor (ILD) is analyzed. It is found that W could be well controlled. The causes of no W accumulation are clarified by analyzing the background plasma parameters and modeling the W transport. It turns out that the electron temperature (T_e) and its gradient are usually high while the toroidal rotation and density peaking of the bulk plasma are small. In this condition, the modeled W turbulent diffusion coefficient is big enough to offset the total turbulent and neoclassical pinch, so that W density profile for zero particle flux will not be very peaked. Combining NEO and TGLF for the W transport coefficient and the impurity transport code STRAHL, not only the core W density profile is predicted but also the radiated information mainly produced by W in the experiment can be closely reconstructed. At last, the physics of controlling W accumulation by electron cyclotron resonance heating (ECRH) is illustrated considering the effects of changed T_e by ECRH on ionization balance and transport of W. It shows that the change of ionization and recombination balance by changed T_e is not enough to explain the experimental observation of W behavior, which should be attributed to the changed W transport. By comparing the W transport coefficients in two kinds of plasmas with different T_e profiles, it is shown that high T_e and its gradient play a key role to generate large turbulent diffusion through increasing the growth rate of linear instability so that W accumulation is prevented.

Illustrating the physics of core tungsten (W) transport in a long-pulse steady-state H-mode discharge on EAST

“…Due to the lack of an experimental W density profile for this shot, we can only compare the W density profiles of other similar experiments and this model in a quantitative way. In fact, the experimental W density profiles in many pure RF-heated H-mode discharges are similar; see, for example, the shots numbered 67682 and 67692 [40], 73886 [32], and 73888 [30], where only the W 43+ , W 45+ densities are measured and are within a magnitude of about 10 14 m 3 . Thus, the W density profiles of our modeled results in figure 12(a) are quantitatively consistent with the experimental results.…”

“…In addition, different W transport behaviors can be studied in diverse plasma scenarios heated by various systems [27]. For example, W accumulation is often observed in the hybrid scenario developed by NBI + LHW [28,29], while W can regularly be well-controlled in the fully non-inductive scenario, mainly using a combination of NBI, LHW, and ECRH [30][31][32]. Nevertheless, these previous W transport studies of the EAST have investigated short-pulse H-mode discharges [29,32].…”

“…For example, W accumulation is often observed in the hybrid scenario developed by NBI + LHW [28,29], while W can regularly be well-controlled in the fully non-inductive scenario, mainly using a combination of NBI, LHW, and ECRH [30][31][32]. Nevertheless, these previous W transport studies of the EAST have investigated short-pulse H-mode discharges [29,32]. Long-pulse steady-state operation is an important goal of the tokamak development program.…”

“…In addition, ICRH can also increase the W source; therefore, it cannot be used to control W accumulation in the EAST at present. ECRH and LHW in the EAST are candidates for accomplishing this mission [32,36]. The work presented in this paper will focus on identifying the dominant physical mechanisms responsible for the core W control in an EAST-ILD pure RF-heated long-pulse steadystate H-mode discharge (#66740) in 2016, and comparing the theoretical models for the description of W transport with the best available experimental W information, including the W concentration (C W ) and the radiated power (P rad ) density profile in the EAST.…”

Core transport modelling of the DTT full power scenario using different fuelling strategies

“…Another element which can influence the density profiles is the location of the power deposition of the Electron Cyclotron Resonance Heating (ECRH) system, which has been selected as main auxiliary heating system for DTT. The well-known density pump-out effect [16], which is beneficial for preventing the impurity accumulation in the plasma core, can significantly modify the density profiles [17][18][19]. On the other hand, the electron cyclotron resonance (ECR) power deposition itself can be influenced by the density perturbation due to pellet injection, because of the dependence of the ECR efficiency and location on the plasma density.…”